X-ray timing apparatus



All@ 12, 1941 R. 1 swEENY ETAL 2,252,530

X-RAY T IMING APPARATUS Filed Sept. 6, 1940 4 Sheets-Sheet 1 Aug. l2,1941.

R. L. swEENY ETAL 2,252,530

X-RAY TIMING APPARATUS Filedept. 6, 1940 4 sheets-sheet' 2 I 771/@ al y.MW

Aug. 12, 1941. R. L. swEENY ETA. l X-RAY TIMING APPARATUS Filed Sept. 6,1940 4 Sheets-Sheet 3 Aug- 12, l941- R. L. swEENY ETAL 2,252,530

XRAY TIMING APPARATUS I Filed Sept. 6, 1940 4 Sheets-Sheet 4 PatentedAug. 12, 1941 X-RAY TIMING APPARATUS Raymond Llewellyn Sweeny, ForestHill Village, Toronto, Ontario, and George Albert Richards, Toronto,Ontario, Canada, assignors to Fen-anti Electric Limited, Toronto,Ontario, Canada Application september 6, 1940, serial No. 355,702

16 Claims. (Cl. Z50-95) The present invention relates to apparatus fortiming X-ray exposures.

In producing a satisfactory X-ray film, two principal factors determinethe resultant quality of the film. The contrast is determined by thevoltage used on the X-ray tube anode, and the density is determined bythe total quantity of X-rays, which can be expressed in terms of themilliampere-second product. Milliamperes and time separately have noimportance to lm density; it is only their product that has significancein this respect. ,Y

The comlmon method of securing acertain desired total number ofmilliampere-seconds` has been to set the filament of the X-ray tube to atemperature which will produce a value of milliamperes anode currentwhich, when multiplied by the time set on the timer, will produce thedesired product of milliampere-seconds. This system has manydisadvantages. tube filament must be set accurately,usuallynecessitating one or more calibrating exposures, which may wastevaluable iilm and which reduces the useful life of an expensive tube.inaccuracies in setting the X-ray timer and milliamperage the use of thepresentinvention, to determine ilm density with one setting instead oftwo, but any fluctuations in line voltage, which would be magnifiedthrough the exponentiall filament characteristic to the anode current,are accurately compensated fo-r. This ensures the production of aproperly exposed lilrn in all cases. Extreme accuracy of filamentsetting is not required, and calibrating exposures are therebyeliminated with corresponding increase in the life of the X-ray tube.

Despite the fact that time is an important factor in some cases, notablyin chest work, this does not reduce the usefulness of applicantsmilliampere-second timer, since a simple calculation will enable theproper time and milliamperage to be determined.

Sin-ce the product of milliamperes and seconds determines density, acalculation is implied in every case when the customary straight timeris used. Applicants milliampere-second timer only requires thiscalculation occasionally and the dominant factor is always set with thesubordinate factor calculated. The straight timer requires the settingof two subordinate factors to secure the desire-d dominant factor.

The X-ray Attempts have been made to provide a milliampere-seconds timerbut so far as applicants are aware no practical and accurate timer ofthis kind has hitherto been proposed.

The prime object of the present invention is to provide an accurate andrelatively simple apparatus for use with an X-ray transformer unit,which will provide such milliampere-seconds timing with great accuracyand uniformity of operation.

A further object of the invention is to provide means' for compensatingfor the non-linearity of the (exponential) charging curve of thecondenser-resistance timing circuit so as to obtain the same operatingcharacteristic as would be obtained with a linear charging curve,thereby making the device accurate over its entire range of operation.

A further object of the invention is to provide means for utilizing avery large portion of the `charging curve of the condenser-resistancetiming circuit so as to allow the use of a condenserresistance timingcircuit having a small timeconstant in such a manner that the voltageappearing across the condenser is relatively large, thereby making `forstable operation of the device. l

A still further object of the invention is to provide .means to allowthe use of the heavy switchgear necessary to control the large primarycurrents whichare used in high-power X-ray apparatus, and to providemeans for compensating for the time lag inherent in the operation ofsuch large heavy switchgear.

A still further object of the invention is to provide an accurateapparatus for use with an X-ray transformer unit, which apparatus willallow exposures to be timed in terms of milliampere-seconds in such amianner that the percentage error in setting the scale will be constant,rather than the amount of error asis the common practice.

A still further object of the invention is to provide means whereby the`apparatus will not operate until the filaments of all electron tubestherein shall have come up to operating temperature, thereby eliminatingthe possibility of operation under conditions which might produceinaccurate results.

According to applicants invention the actual X-ray tube anode current isutilized to actuate the timing device. The property whereby a condenserneeds a certain definite time to acquire a certain definite charge isutilized to form the timing element. Various methods to make use of thisproperty are hereinafter described.

The invention consists in the combination and arrangements of elementshereinafter described and more particularly pointed out in the appendedclaims.

Referring now to the accompanying drawings which illustratediagrammatically, and by way of example only, various embodiments of theinvention:

Figure 1 is a diagram illustrating the principle of the invention,

Figure 2 is a diagram similar to Figure l but showing an arrangement inwhich the use of batteries is eliminated,

Figures 3 and 4 are diagrams of detail modiiications,

Figure 5 is a diagram of a preferred form of the invention for use witha half-wave X-ray machine.

Figure 6 is a diagram of a preferred form of the invention for use witha full-wave X-ray machine.

In Figure 1 the numeral I indicates the alternating current supply whichis connected to the primary 2 of the X-ray transformer T through aswitch 3 of the X-ray contactor 4 which includes a coil 5. The secondary6 of the transformer T is connected at one end to the cathode 'I of theX- ray tube 8 and at the other end to the anode 9 and to ground througha resistance R1. 'I'he top of .the resistance R1 is connected to thegrid of a control tube I through resistances R2 and R3, and the lowerend of the resistance R1 is connected to the cathode of the tube I0through a grid bias II. A timing condenser C is connected at one side toa point between the resistances R2 and R3 and at the other side to theanode of the X-ray tube 8. A switch I2 is connected in parallel with thecondenser C and preferably a condenser C1 is bridged across theconnections to the grid and cathode of the control tube IB. The platebattery, shown at I3, is connected to the plate of the tube III throughone side of a switch I4 and the coil of a relay I5.

One of the supply lines I can be connected to the other through theother side of the switch I4, the movable element of the relay I and thecoil 5 of the contactor 4.

Resistance Rz is preferably large with respect to resistance R1. Thecontrol tube I0 may be of either hot cathode or cold cathode type withan appropriate type of xed bias of such value that with switch I2closed, the tube I0 is in its nonconducting phase. Resistance R3 is usedto prevent the ow of excessive grid current when the tube I0 isconducting.

To obtain an X-ray exposure it is necessary either rst to open switch I2and then close switch I4 or to perform these two operationssimultaneously, preferably using a suitable switch or relay as will belater described.

On closing switch I4, the coil 5 of the contactor 4 is energized toclose the contactor switch 3 so that the exposure is thus initiated.Simultaneously, the closure of switch I4 also applies plate voltage tothe tube IU through the coil of relay I5. Thus far the tube I0 isnon-conducting.

The X-ray tube current now flows through resistance R1 in such adirection as to cause a voltage drop Ex of the polarity shown. Exappears The voltage Ee, across condenser C, increases exponentially withtime, and appears as a component of the total grid bias on the tube Il),acting in opposition to the xed bias I I.

Instead of applying voltage Ec directly in the bias circuit of the tubeI0 it can be applied to the grid of a D. C. amplier of appropriatedesign, not shown, so that the output of this amplier then acts in thegrid circuit of the tube I0. Thus as voltage Ec increases, the totalresultant bias decreases, and on reaching a predetermined value, thebias becomes insufcient to prevent the flow of anode current, and so thetube II) begins to pass plate current. This plate current energizes thecoil of the normally-closed relay I5, causing it to open, therebyautomatically opening the circuit of the contactor coil 5, so that theswitch element 3 opens and thus terminates the exposure. At the end ofthe exposure switch I4 is opened and switch l2 is closed and the closingof the latter switch discharges the timing condenser C in preparationfor the next exposure.

With the arrangement just described, the condenser C1 in the gridcircuit of the tube I0 was found to be necessary to prevent surgeimpulses produced in closing switch I4 from reaching the grid.

The value of milliampere-seconds to be obtained can be determined by anadjustment of resistance R1 or R2 or condenser C, any one of which willaiect the time required for the charge voltage E@ to build up across thetiming condenser C to the required amount to cause the tube I 0 to passplate current.

Other methods of determining Values of milliampere-seconds to beobtained are varying the grid bias voltage, anode voltage, or even thecathode heating current (in the case of a hot cathode control tube),thus changing the amount of charging voltage E@ required to operate thetube I0. These methods, however, are not as satisfactory as thosedescribed in the preceding paragraph.

A triode, tetrode, or other multi-element vacuum tube may be used inplace of a gaseous discharge, grid controlled tube as the control tubeI0. With the high vacuum type of control tube the action is slightlydifferent. In that case the plate current builds up simultaneously withthe condenser charge voltage Ec until it reaches such a value that itwill be suiiicient to cause the normally closed relay I5 to open. Insuch an arrangement it is found feasible to determine themilliampere-seconds value by adjusting the spring tension of the relayI5, as well as by any of the previously described methods.

Another method of using the vacuum type electronic tube is to allow thistube to be conducting during the exposure, operating a normally-openrelay (or even operating the X-ray contactor 4 directly). In this casevoltage Ec is added to the iixed bias in such a manner that the two areadditive, so that as E@ increases, the vacuum type control tube currentfalls, finally reaching a value low enough to allow the relay orcontactor to open.

For accurate operation of the circuit shown in Figure 1, it is assumedthat the condenser voltage Ec increases in a linear relationship withtime, assuming the voltage EX is constant. Actually this increase isexponential, but if the operation of the circuit in Figure 1 is confinedto a small portion of the beginning of the curve, then the increase isalmost linear and reasonably accurate operation is obtained. In order tosecure substantially linear operationV by utilizingr this small initialportion of the curve, itl isY necessary to make the time constant ofresistance R2 multiplied by the capacity of condenser C quite large.

This product (RZXC) should be numerically about 3 times as great as thedesired longest ex-v posure, measuredl in seconds. For instance, supposeit is desired to time a maximum exposure of 20 seconds, then RZXC shouldbe:

If we let :10 microfarads, then-.R2 must be:-

=6,000,000 ohms or 6 ncegohms` Where tztime in` seconds xzX-ray tubecurrent. in milliarnperes K=arbitrary constant.

Since IX isin milliamperes andi t in seconds, this means that under theabove conditions the value of milliampere-seconds obtained is consta-nt,regardless of the magnitude of the X-ray tube current Ix.

By changing any of the circuit conditions, as previously described) itis possible' to secure any desired'value of milliampere-seconds.

In practice, it is desirablev toeliminateI batteries as a sourceof-Voltage supply, and to design the device for operation froml anyconvenient alternating current supply service. An alternating currentform of the circuit shown in Figure 1 is illustrated in Figure 2 inwhich the corresponding elements are indicated by the sa-me numerals asin Fig-ure l as` far as possible. The supply leads lA are connectedV toYthe' primary 2 of a transformer T`, and the filament of the tube Il) isconnected toy a secondary coil Ea. A secondary coilA 6b' is connected tothe filament cfa full- Wave rectifier tube- IB" the plates of whichL areconnected to the ends of a secondary coil 5C. The middle point of thesecondary 6a is connected to an adjustable tapv at I'l to a resistancet8 which is connected to the centre points of secondaries (5b4 and 5c inparallel with a filter unit consisting of a choke coil L and condensersC3 and C4. The ends of the resistance IB`` are also connectedrespectively tothe grid", and to the plate of the tube Ii)v through theswitch M and the coil of the relay i5.

In this particular arrangement, bias is obtained by making the cathodeof the controlr tube IU positive (rather than the grid negative) withrespect tothe common lead. Alternating current heating current is"supplied to the filament and its electrical centre is obtained by meansof the centrer-tapped winding 6a or if desired by'means of avcentre-tapped resistor across the windingV as is common practice.

D. C. for the anodeandy grid of control tube t0" is obtained from theoutput` of the filtered fullwave rectifierY system I6, C3, C4, L, T.

As previously stated, the curve of condenser chargel voltage againsttime `is not the ideal straight linegbut an exponential curve. By us ingaI combination of resistance R2 and capacity C with alargetime-constant, a straight line isr quite closely approximated, but thisarrangement has the disadvantage? that the valve of Ec necessary tocause the control tube I0 toi pass plate current is necessarily small,so longas the range of EX is kept down within reasonable limits. Sincesuch a small variation in gridV bia-s is depended` on to cause thecontrol tube It to'function, operation becomes somewhat critical, and.quite sensitive to changes in tubeA characteristics, line voltagefluctuation, and theV like, By de-y creasing theA time-constant ofthe--RzXC combination it is therefore possible toimprove the sta--bility of operation',l but at the sacriiice of true linearity' ofoperation. With aY small time-constant combination the ratio of` long toshort exposures for any given milliampere-seconds set ting is;` tooAgreat.

To overcome this, a circuit element may be added, in the form of a smalliixed voltage' supe ply inseries with This additional voltage issupplied forexample', from a tap at 2l]L on the same D. C. supply (seeresistance i8, Figure 2)v asis used for the plate and grid circuits oftheq control tube I, and is so connected that it adds to` the voltage EXas shown in. Figure 3.

This E. M. F., E1 being fixed, will' contribute an increasing proportionof the condenser charging voltageV as Ex decreases. E1 is only afraction ofY one percent of the maximum value of Ex (that is` during.the shortest exposure) but amounts to possibly fty percent of theminimum value of EX (that is during the longest exposure)'. Thus Er actstoV shorten the long exposures, and` has a negligible effect on the veryshort exposures.

By properly adjusting E1 it is thus possible to obtain almost perfectlinearity over a whole range ofv exposures up to about 1.3time-constants, for instance, with C equalling IO'micro-farads and R2Vequalling 6 megohms, it would be possible to time up to' about l.360'=78 seconds.

One feature that materially limits the sphere of accurate operation ofany true milliampereseconds timer is the lengthening ot the very shortexposures due to the slow action of most contactors in opening thecircuit. With a milliampere-seconds timer the timing action does notbegin until the contactor is closed, so that the time required forclosure is of no consequence. However, at the end of the exposure,A whenthe timer cuts 01T the contacter coil current, the' X-rayV currentcontinues to flow until the contactor has opened. With the timerproviding anv exposure of 1/25 second, and a contacter lag of j/scsecond, say, an overall exposure time of 3/50 second is Obtained, or a-nerror of 50%.

With the circuit rnodiiication` shown in Figure 4 the lengths of allexposures provided by the' timer can be shortened, by equal amounts.`

This lag compensator circuit consists of a resistor R4 and a condenserC2 connected in parallel between the resistance R2 and timing condenserC. With this arrangement, the E. M. F. appearling at the grid of thecontrol tube IU is the sum of the xed bias, the condenser charge volt`-age, and the IR drop in resistance R4. The rate' of increase of En isdirectly proportional to the'v current through resistance R4, and hence,tothe IR drop across resistance R4. Also, during any given exposure, agiven interval of time, say 1go second, is represented and accompaniedby a certain increase in the Value of Ec. The amount of this increase isdirectly proportional to the IR drop in resistance R4. Thus, when thislatter IR drop is added in series with Ec, the length of all exposureswill be decreased by a constant amount. By adjusting resistance R4 allexposures can be shortened by an amount equal to the contactor openinglag, and thus compensation can be obtained for this lag. The condenserC2, is included to lter out any high peaks which may occur in thedistorted X-ray current wave form. Without this condenser operation maybe somewhat unstable.

A preferred form of the improved timer, for use in conjunction with ahalf-wave X-ray machine, is shown in Figure 5, including all the circuitfeatures thus far discussed.

In this arrangement, resistance R1 is used as the milliampere-secondscontrol. Resistance R2 is arranged in a suitable number of sections andis used as a means of changing the range of operation which is set bymeans of a range switch i 2I. In place of switches I2 and I4 shown inFigures 1 and 2, the desired operation is obtained by using apush-button or foot-switch 22 and a second relay 23. The relay 23comprises a coil 24 shunted by a condenser C5 and connected to oppositeends of the resistance I8 through the push-button 22. The relay 23 alsocomprises a pair of blades 25 and 26, the former being normally closedand coacting with a contact connected to the grid of the tube IQ and thelatter being normally open and coacting with a contact connected to themovable element of relay I5.

The secondary of the X-ray transformer T2 is divided into two parts 2!and 28 one of which is connected to the filament 'I' and to theresistance R1 through a direct current milliammeter 2S, while the otheris connected to the anode 9 and to ground.

The primary 2 of transformer T2 is connected across the A. C. source Ithrough the normally open contactor relay 4, the source being providedwith a switch 36. The source I is also connected to the primary of atransformer T1, the secondary of which comprises three windings 3|, 32and 33. spectively to the middle points of secondaries 32 and 33 througha lter consisting of a choke coil L and condensers C3 and C4. The endsof winding 32 are connected to the lament, and the ends of winding 33 tothe plates of a rectifier tube 35. The ends of winding 3| are connectedto the lament of control tube II) and the middle point of winding 3Ileads to a tap 36 on resistance I8. The lower end `of resistance R1leads to a tap 3l on the resistance I8, to supply the fixed voltage E1.(See Figure 3.)

'Ihe following is a list of suitable values for the various elements inFigure 5, though the invention of course, is not limited to such values:

R11,000 ohms variable (3l-.001 microfarad Rg--Range l-l00,000 ohms Cz-LOmicrofarad Range 2500,000 ohms Range 3-2.5 megohms Ita-400,000 ohmsR4-5,000 ohms variable 18 5,000 ohms total C-4.0 microi'arads C14-4.0microfarads (l5-0.25 microfarad C 10.0 mierofarads' L- hcnries By way ofexample the voltage drop across winding 3| may be 2.5 volts, acrosswinding 32, 5 volts, and across each half of Winding 33, 125 volts. TubeI0 may conveniently be of type 2A4G and tube 35 of type 82.

The ends of resistance IB are connected ref The operation of this formof the invention is as follows:

The line switch 39 is closed to connect the whole apparatus to the A. C.line. The transformer T1 is now energized and so supplies filamentheating current to the control ltube I!) and the rectifier tube 35 inthe timer. Transformer T1 also supplies A. C. to the plates of therectifier tube 35, and the latter converts the A. C. to pulsating D. C.The pulsating D. C. is ltered by unit L, C3, C4, thus givingsubstantially pure D. C. across C4 and I8, having the polarityindicated. This pure D. C. is required to supply the plate and gridcircuits of the control tube I0.

It Will be noticed that with relay 23 in the normal position shown, thegrid of control tube I0 is connected through resistance R3 to thenegative side of the D. C. supply, whereas the lament of control tube I8is connected, through the transformer winding 3l, to the tap 36 onresistance I8, thus making the lament positive with respect to the grid.In this way a fixed grid bias is obtained.

As yet no anode current flows in the X-ray tube 8, since transformer T2is not yet energized. We do assume, however, that the filament of theX-ray tube is appropriately heated at this point.

Next the exposure push-button 22 is pressed to connect the D. C. supplydirectly to the coil 24 of relay 23 and at the same time to the plate ofthe control tube I 0 through the coil of relay I5. The fixed grid biason the control tube is greater than the critical bias at this point, andso no plate current flows as yet. This closure of button 22 operatesrelay 23 and energizes the coil 5 of the heavy primary contactor 4 tocause closure of the normally-open contact 3. 'Ihe normally closedcontact of relay 23 has, until now, acted as a short circuit across thecombination of C, R4 and C2. This short-circuit is now opened. Tosimplify explanation, the lag compensator circuit which consists of R4and C2 may, for the time being, be regarded as short circuited.

The contactor 3, 4, 5 is a heavy device with large contacts usuallyimmersed in oil to carry the heavy primary current of the X-raytransformer. Although similar in diagrammatic representation, it mustnot be confused with the very much smaller and lighter relays I5 and 23,which are parts of the timer itself.

On the closure of the primary contactor 3 the X-ray transformer T2 isenergized and so X-ray anode current is caused to flow.

Since current is now flowing in resistancce R1 there is a voltage dropacross it proportional to the current flowing and having the polarityindicated. To this voltage (Ex in Figure 3) is added the voltage (Er inFigure 3) in the small portion of resistance I8 between the lower tap 31and ground, which drop is caused by the X-ray anode current plus thecurrent flowing in resistance I8 from the rectier tube 35.

Ex plu-s E1 now appears across the timing circuit (consisting ofresistance R2 and condenser C in series) and the condenser C begins tocharge up at a rate depending on the values of R2 and C, and the voltageEX plus Er. I'he charge Voltage across C is of the polarity shown and soacts in opposition to the fixed bias voltage.

When the grid bias on the control tube II) is decreased by thepredetermined amount the point of critical bias is reached, the controltube ignites, and the plate current in the control tube I Il flows andenergizes the coil of relay I5. The

normally-closed contacts of this relay now open, thus de-energizing thecoil of the primary contactor 4, thereby opening the primary circuit ofthe X-ray transformer T1, and ending the exposure.

In the ideal arrangement the exposure would end at the instant whencondenser C' became charged up to the point of critical bias, whereas itwill be seen that there are three steps that follow this, namely,ignition of tube I0, opening of relay I and opening of contactor 3. Thefirst can be considered to take zero time, but the opening of relay I5and especially the opening of contactor 3 take measurable periods andactually add to the exposure time. For this reason the lag compensatorcircuit, comprising resistance R4 and condenser C2 was devised, and theoperation of this circuit has already been described with reference toFigure 4.

The lower tap 31 on resistance I8 is for the curve compensator, whichhas been described with reference to Figure 3.

After the exposure is completed the push-button 22 is released. Thisde-energizes the relays I5 and 23 so that they return to their normalpositions shown in Figure 5. The normallyclosed contact of relay 23 isused to short-circuit timing condenser C to remove its charge, inpreparation for a subsequent exposure.

In the full-wave circuit shown in Figure 6 the secondary of transformerT2 is connected to the X-ray tube through a bridge rectifier circuitindicated at 38.

Transformer winding 21 is connected to resistance R1 through an A. C.milliammeter 23. The lower end of resistance R1 is tapped to curvecompensator resistance 39 which is grounded at one end and connected atthe other end to ground through a secondary winding 40 of transformerT1.

The top of resistance R1 and the bottom of resistance 39 are eachconnected to sets of resistances R5 and Re provided with range selectorswitches 4I, which are connected as indicated to double diode tubes 42,which are connected on one side to the bottom of condenser C and to thenormally-closed contact 25 of relay 23, and on the .other side to thelower end of resistance I8.

The top of the lag compensator circuit R4, C2 is connected to a tap 43near the bottom of resistance I8 for cancellation of Contact E. M. F.developed in the double diode rectifier tubes.

The relay coil 24 is connected to the bottom of resistance I8 through aslow heating rectifier tube An electrostatic shield may be provided intransformer T1 as indicated at 45.

The following suitable values of the elements of Figure 6 are given byway of example only:

SR1-1,000 ohms variable (Jr-.001 microfarad 'Rn-100,000 ohms Cz-LOmicrofarad lil-5,000 ohms variable (la-4.0 microiarads Rs-Range 1-50,000ohms each (3l-4.0 microfarads and Range 2-250,000 ohms each Rrr-Range3-1.25 megohms each C15-0.25 microfarad 18 5,000 ohms total C -l0.0microfarads L =30 hennes exposure before this heating period haselapsed. This is done by providing, in the coil circuit of relay 23 theslow-heating rectifier tube 44.

One more point of difference between the two circuits is in thecurve-compensator arrangement. In the half-wave form shown in Figure 5 atap 31 on resistance I3 was used to supply D. C. which was added to theD. C. drop in resistance R1. In the full-wave timer, since there is A.C. flowing in resistance I3, it is added to an A. C. voltage obtainedfrom the additional winding 43 on transformer T1.

For simplicity, in Figures 5 and 6 the primaries -of both transformersT1 and T2 are both shown as being connected to al single A. C. sourceand this arrangement can be used in practice when desired, but a commonarrangement is to connect the two transformers to separate sources, forexample transformers T1 to a supply -at volts and transformer T2 to asupply at 220 Volts.

It will be understood that the improved timer may be used in conjunctionwith any type of X-ray machine, whether the X-ray filament heating isadjusted by a continuously adjustable resistance or choke or by tappedresistances or otherwise.

It will also be understood that the lag compensating means will berequired in the -timer only when a contactor with relatively slow-movingparts is employed in the X-ray machine. Certain X-ray machines employ anelectronic relay system in place of a mechanical contactor, and in suchcases lag compensating devices are unnecessary.

The foregoing description and accompanying drawings are given by way ofexample only and any modifications within the scope of the appendedclaims may be resorted to without departing from the spirit of theinvention.

What we claim is:

1. Apparatus for timing X-ray exposures cornprising a low resistance tobe connected in the secondary circuit of the X-ray tube high-tensiontransformer, a timing circuit including a relatively high resistance anda condenser in series and connected in parallel with said' lowresistance, a control tube having a voltage-sensitive element connectedto said condenser so as to respond to changes in the voltage across saidcondenser, switching and relay means connected with said control tubeand with the primary winding of the X-ray high-tension transformer tocause automatic opening of said primary winding when said condenser hasattained a predetermined charge, and means for compensating for thenon-linear charge curve of the timing condenser.

2. Apparatus as claimed in claim 1, wherein switching means are providedfor short-circuiting said timing circuit after each exposure.

3. Apparatus as claimed in claim l, wherein switching means are providedfor automatically short-circuiting said timing circuit after eachexposure.

4. Apparatus as claimed in claim 1, wherein a second high resistance isinserted between said timing condenser and the voltage-sensitive element-of the control tube.

5. Apparatus as claimed in claim 1, wherein a second high resistance isinserted between said timing condenser and the voltage-sensitive elementof the control tube, the timing condenser abeing bridged by a relativelylow capacity condenser at a point between said second high resistanceand the voltage-sensitive element of the control tube.

6. Apparatus as claimed in claim 1, wherein said timing circuit includesalso a rectifier connected between the relatively high resistance andtiming condenser so that a unidirectional voltage is applied to saidtiming condenser.

7. Apparatus as claimed in claim 1, wherein means are provided forcompensatingT for time lag caused by delay in the opening of relativelyslow-moving parts of the circuit.

8. Apparatus as claimed in claim 1, wherein means are provided forcompensating for time lag caused by delay in the opening of relativelyslow-moving parts of the circuit, said means comprising a resistance anda condenser connected in parallel with each other and in series with thetiming condenser.

9. Apparatus as claimed in claim 1. wherein said means compensating forthe non-linear charge curve of the timing condenser, consist of meansfor adding a suitable fixed voltage to the voltage app-lied to thetiming circuit by the voltage drop across the low resistance.

10. Apparatus as claimed in claim 1 wherein the high resistance in thetiming circuit is arranged in a plurality of parallel sectionscooperating with a range selector switch.

11. Apparatus as claimed in claim 1, comprising also -a low-tensiontransformer, a resistance connected with secondary windings of saidtransformer, a rectifier between said windings and said resistance, alter unit between said rectifier unit and said resistance, a normallyopen relay connected across said resistance through an exposure switchand a normally closed relay having a switch connected to said normallyopen relay and a coil connected to the plate of the control tube and tosaid exposure switch, said normally closed relay being also adapted tobe connected to a normally open relay in the primary circuit. of thehigh-tension transformer of the X-ray tube.

12. Apparatus as claimed in claim 1, comprising also a low-tensiontransformer, a resistance connected with secondary windings ofsaidtransformer, a rectifier between said windings; and said resistance,a filter unit between said rectifier u nit and said resistance, `anormally open relay connected across said resistance through an exposureswitch and a normally closed relay having Ya switch connected to sai-dnormally open relay and a coil connected to 'the plate of .the controltube and to said exposure switch, said normally closed relay being alsoadapted to be connected to a normally open relay in the primary circuitof the high-tension transformer of the X-ray tube said normally openrelay also comprising a normally closed switch arranged to short-circuitthe timing circuit after each exposure.

13. Apparatus as claimed in claim 1, comprising also a low-tensiontransformer, a resistance connected with secondary windings of saidtransformer, a rectifier between said windings and said resistance, afilter unit between said rectifier unit vand said resistance, a normallyopen relay connected across said resistance through an exposure switchand a normally closed relay having a switch connected to said normallyopen relay and a coil connected to the plate of the control tube and tosaid exposure switch, said normally closed relay being also adapted tobe connected to a normally open relay in the primary circuit of thehigh-tension transformer of the X-ray tube the low resistance connectedin the secondary circuit of the high-tension transformer being connectedto a point adjacent the negative end of the resistance connected withthe low-tension transformer.

14. Apparatus as claimed in claim 1, comprising `also a low-tensiontransformer, a resistance connected with secondary windings of said.transformer, a rectifier between said windings and said resistance, afilter unit between said rectier unit and said resistance, a normallyopen relay connected across said resistance through an exposure switchand a normally closed relay having a switch connected to said normallyopen relay and a coil connected to the plate of the control tube and tosaid exposure switch, said normally closed relay being also adapted tobe connected to a normally open relay in the primary circuit of thehigh-tension transformer of the X-ray tube the ends of a secondarywinding of the low-tension transformer being connected to the cathode ofthe control tube and the centre point of said winding being connected tothe resistance connected with the low-tension transformer.

15. Apparatus as claimed in claim 1, comprising also a low-tensiontransformer, a resistance connected with secondary windings of said`transformer, a rectifier between said windings and said resistance, alter unit between said rectier unit and said resistance, a normally openrelay connected across said resistance through an exposure switch and anormally closed relay having a switch connected to said normally openrelay and a coil connected to the plate of the control tube and to saidexposure switch, said normally closed relay being also adapted to beconnected to a normally open relay in the primary circuit of thehigh-tension transformer of the X-ray tube a slow-heating rectifier tubebeing interposed between the coil of said normally-open relay and theresistance connected with said low-tension transformer.

16. Apparatus as claimed in claim l, comprising also a low-tensiontransformer, a resistance connected with secondary windings of saidtransformer, a rectiiier between said windings and said resistance, afilter unit between said rectifier unit and said resistance, a normallyopen relay connected across said resistance through an exposure switchand a normally closed relay havinga switch connected to said normallyopen relay and a coil connected to the plate of the control tube and tosaid exposure switch, said normally closed relay being also adapted tobe connected to a normally open relay in the primary circuit of thehigh-tension transformer of the X-ray tube electrostatic shielding beingprovidedvin the low-tension transformer.

RAYMOND LLEWELLYN SWEENY. GEORGE ALBERT RICHARDS.

